Test plates for chemical or biochemical analyses, or sample preparation and purification, which contain a plurality of individual wells or reaction chambers, are well-known laboratory tools. Such devices have been employed for a broad variety of purposes and assays, and are illustrated in U.S. Pat. Nos. 4,734,192 and 5,009,780, 5,141,719 for example. Microporous membrane filters and filtration devices containing the same have become particularly useful with many of the recently developed cell and tissue culture techniques and assays, especially in the fields of virology and immunology. Multiwell plates, used in assays, often utilize a vacuum applied to the underside of the membrane as the driving force to generate fluid flow through the membrane. Centrifugation also can be used as the driving force. The microplate format has been used as a convenient format for plate processing such as pipetting, washing, shaking, detecting, storing, etc.
Typically, a 96-well filtration plate is used to conduct multiple assays or purifications simultaneously. In the case of multiwell products, a membrane is placed on the bottom of each of the wells, or a single membrane extends across all of the wells. The membrane has specific properties selected to separate different molecules by filtration or to support biological or chemical reactions. High throughput applications, such as DNA sequencing, PCR product cleanup, plasmid preparation, drug screening and sample binding and elution require products that perform consistently and effectively.
One such filtration device commercially available from Millipore Corporation under the name “Multiscreen®” is a 96-well filter plate that can be loaded with adsorptive materials, filter materials or particles. The Multiscreen® underdrain has been processed in such a way in order to facilitate the release of droplets. More specifically, the MultiScreen® underdrain includes a spout for filtrate collection. This spout not only directs the droplets but also controls the size of the droplets. Without this underdrain system, very large drops form across the entire underside of the membrane and can cause contamination of individual wells. Access to the membrane can be had by removing the underdrain. However, the device is not compatible with automated robotics equipment such as liquid handlers, stackers, grippers and bar code readers.
The Society for Biomolecular Screening (SBS) has published certain dimensional guidelines for microplates in response to the non-uniformity of commercial products. Specifically, the dimensions of microplates produced by different vendors varied, causing numerous problems when microplates were to be used in automated laboratory instrumentation. The SBS guidelines address these variances by providing dimensional limits for microplates intended for automation.
In embodiments where the underdrain is removable, occasionally the underdrain can disengage from one or more wells, resulting in leakage. This is more likely to occur when the buffer dries in the underdrain spout and blocks the passage of the filtrate, as the resulting build-up of pressure ultimately can cause the underdrain to “pop-off” one or more wells. In addition, if the underdrain does not sit flat against the grid or other support surface used in a vacuum manifold, local disengagement can occur upon application of vacuum, again resulting in undesirable leakage between the underdrain and the plate.